1. Field of the Invention
This invention relates generally to laser systems and, more particularly, to optical systems for producing coherent, ultraviolet radiation.
2. Description of the Related Art
As technology progresses, there is an increasing demand for ever more powerful integrated circuits or, equivalently, a demand to include ever more circuitry into silicon chips that form integrated circuits. The result is that the circuits are reduced to ever smaller dimensions, requiring that ever finer features must be patterned during the manufacturing process. In order to meet this demand, the microlithography tools which are used to pattern such fine features have also been required to operate at ever shorter wavelengths, with the most recent generation of tools moving toward lasers that produce radiation beams with wavelengths in the ultraviolet region of the electromagnetic spectrum.
Excimer lasers are commonly used to produce the ultraviolet beams required by such microlithography tools. Specifically, the krypton fluoride (KrF) laser is commonly used to produce ultraviolet radiation at a wavelength of 248 nanometers (nm) (i.e., in the deep ultraviolet or DUV portion of the electromagnetic spectrum); the argon fluoride (ArF) laser is commonly used to produce radiation at a wavelength of 193 nm (i.e., also in the DUV); and the fluorine (F2) laser is commonly used to produce radiation at a wavelength of 157 nm (i.e., in the vacuum ultraviolet or VUV). These excimer lasers can produce ultraviolet radiation suitable for the patterning of fine features. As a result, a significant amount of work has occurred toward producing microlithography tools based on these lasers.
However, excimer lasers also suffer from significant drawbacks. For example, excimer lasers typically are expensive and require constant maintenance due to contamination produced by the laser discharge. In addition, the radiation produced by these lasers is fixed at certain wavelengths (i.e., excimer lasers generally are not wavelength tunable) and the beams produced can be of low quality. Excimer lasers typically cannot produce pulses at repetition rates greater than about 2 kHz, making them unsuitable for applications which require high pulse repetition rates. In addition, for applications which require that the laser deliver radiant energy at a certain rate, an excimer laser typically will use a small number of high power pulses rather than a larger number of lower power pulses due to the repetition rate limitation. However, this can lead to damage and/or reduced life for optics in the laser system due to the high peak powers of each pulse.
Hence, there is a need for alternate sources of coherent, ultraviolet radiation, both as an alternative and as a complement to excimer lasers. For example, a source capable of producing ultraviolet pulses at high repetition rates would be a viable alternative to excimer lasers for certain applications. Similarly, a lower power ultraviolet system would also be a viable alternative to excimer lasers for certain applications, particularly if the lower power system had other advantages such as lower cost, higher quality beams and/or simpler maintenance. Even in cases where an excimer laser is a good choice for a particular microlithography application, the application itself may generate an ancillary demand for alternate sources at similar wavelengths to complement the excimer laser. For example, components used in the microlithography application may need to be inspected at the same ultraviolet wavelengths at which they will be used. Optics used in the microlithographic application may be interferometrically tested for quality; photomasks may be inspected and/or measured; and photoresist may be tested for exposure characteristics, all at the same ultraviolet wavelength at which they are intended to be used. For various reasons, sources other than excimer lasers may be preferred for these ancillary tasks.
Systems based on frequency-shifted dye lasers are one alternative source to excimer lasers. Dye lasers, however, have significant drawbacks which limit their practicality. Dyes, being liquids, are inherently messier than, for example, solid state devices. Dye lasers are also more complex than, for example, solid state lasers because the dyes typically require a pumping system to circulate the dyes within the laser cavity. Furthermore, dyes and/or the solvents used with them may be environmentally hazardous, thus requiring appropriate procedures for handling and disposal.
Systems based on solid state lasers are also capable of generating radiation at ultraviolet wavelengths. However, systems such as Alexandrite lasers typically are not capable of repetition rates higher than 20 Hz. In addition, many solid state systems typically rely on multiple stages in which a non-linear crystal is used to shift the wavelength of incoming radiation. However, most such systems require a large number of stages in order to achieve the desired ultraviolet wavelength, and this is undesirable because each stage adds complexity and reduces the optical efficiency of the overall system.
Thus, there is a need for optical systems which can produce ultraviolet radiation, particularly at wavelengths similar to those produced by excimer lasers, but which additionally overcome some or all of the shortcomings discussed above.
In accordance with the present invention, an optical system for producing a coherent beam of ultraviolet radiation includes an optical source, an optical parametric oscillator (OPO), a frequency doubler, and a mixer. The optical source produces a first beam of coherent radiation. The OPO is disposed to receive a first portion of the first beam of radiation to produce a second beam of radiation from the first portion. The frequency doubler is disposed to receive a second portion of the first beam of radiation to produce a third beam of radiation as a second harmonic of the second portion. The mixer is disposed to receive the second and third beams of radiation to produce therefrom a fourth ultraviolet beam of radiation.
In a preferred embodiment, the optical source includes a Nd:YAG laser and a second frequency doubler which frequency doubles the output of the Nd:YAG laser to produce the first beam of radiation at a wavelength of about 532 nm. The OPO produces a second beam of radiation at a wavelength of about 703 nm; while the frequency doubler doubles the 532 nm beam of radiation to produce the third beam of radiation at a wavelength of about 266 nm. The mixer includes a sum frequency mixer which combines the 266 nm and 703 nm beams to produce the fourth beam at a wavelength of about 193 nm, similar to the ArF line of excimer lasers.
In another preferred embodiment, the optical source includes an amplified Nd:YAG-based microchip laser and a second frequency doubler which frequency doubles the output of the amplified microchip laser to produce the first beam of radiation at a wavelength of about 532 nm. The OPO is injection seeded and produces a second beam of radiation at a wavelength of about 851 nm; while the frequency doubler again produces the third beam of radiation at a wavelength of about 266 nm. The mixer includes a four wave difference frequency mixer which combines the second and third beams of radiation to produce the fourth beam at a wavelength of about 157 nm, similar to the fluorine line of excimer lasers. In this embodiment, the microchip laser produces a narrow linewidth output and injection seeding the OPO narrows the linewidth of the beam of radiation produced by the OPO. This is beneficial because the resulting 157 nm fourth beam of radiation will also have a narrow linewidth and corresponding long coherence length. Furthermore, the microchip laser can produce pulses at a high repetition rate, meaning that the overall system will be capable of producing 157 nm pulses at a high repetition rate.
In further accordance with the present invention, a method for producing a coherent beam of ultraviolet radiation includes producing a first beam of coherent radiation and then producing a second beam of radiation from a first portion of the first beam of radiation in an optical parametric process. A third beam of radiation is produced as a second harmonic of a second portion of the first beam of radiation. The second and third beams of radiation are mixed to produce a fourth ultraviolet beam of radiation.
The present invention is particularly advantageous because it only uses three stages (i.e., the OPO, the frequency doubler, and the mixer) to produce ultraviolet beams. This reduces the complexity and cost of the overall approach and increases the overall efficiency. The present invention is also suitable for use with solid state lasers as the optical source, resulting in the general advantages which solid state lasers have over both dye lasers and gas lasers. As another advantage, various embodiments of the present invention may be designed to achieve high repetition rates and/or to produce high quality ultraviolet beams. The use of the OPO, which is wavelength tunable, results in a fourth ultraviolet beam which is also wavelength tunable.
In another aspect of the present invention, an optical system for producing a coherent beam of ultraviolet radiation includes a laser, a fifth harmonic generator, and a Raman shifter. The laser produces a first beam of radiation. The fifth harmonic generator is disposed to receive the first beam of radiation for producing a second beam of radiation at a fifth harmonic of the first beam of radiation. The Raman shifter is disposed to receive the second beam of radiation and anti-Stokes shifts the second beam of radiation to produce a third beam of ultraviolet radiation. In a preferred embodiment, the laser is a Nd:YAG laser which produces the first beam of radiation at a wavelength about 1064 nm and the Raman shifter produces the third beam of radiation at a wavelength about 157 nm, similar to the fluorine line of excimer lasers.
In further accordance with the present invention, a method for producing a coherent beam of ultraviolet radiation comprises the following steps. A first beam of coherent radiation is produced. A second beam of radiation is produced at a fifth harmonic of the first beam of radiation. A third beam of ultraviolet radiation is produced by anti-Stokes shifting the second beam of radiation.
This aspect of the present invention is particularly advantageous because of its simplicity. The result is a low-cost approach to producing radiation at a wavelength close to the fluorine line of excimer lasers.